N-Phthaloyl-L-threonines (P-Thrs) and their dehydrated phthalimides (2-phthalimido-2-butenoic acids, P-BTEAs) were readily synthesized and showed a strong root growth-promoting activity for rice seedlings. Of the synthetic P-Thrs and P-BTEAs, (Z)-2-phthalimido-2-butenoic acid (P-BTEA, 6a), (Z)-2-(4-chlorophthalimido)-2-butenoic acid (4-C1P-BTEA, 6b) and (Z)-2-(3-fluorophthalimido)-2-butenoic acid (3-FP-BTEA, 6c), in particular, had a strong root growth-promoting activity for rice seedlings. This activity at 1X10-4M was 3.3-to 3. 7-fold the control value. The root growth-promoting activities for rice seedlings of all the dehydrated phthalimides (P-BTEAs) were greater than those of the mother compounds (P-Thrs). For lettuce seedlings 4-C1P-BTEA (6b) and 4-MeP-BTEA (6e) alone had a weak root growth-promoting activity at 1X10-4M. Some P-Thrs and BTEAs caused severe necrosis of lettuce seedling roots and hypocotyls at the high concentration of 1X10-3M. Nitro-substituted Thrs and BTEAs with strong electron withdrawing group, however, have no root growth-promoting activity.